The human conceptual repertoire is beyond the grasp of any other animal. How knowledge is acquired is an outstanding scientific mystery, one with profound consequences for human well being. Recent advances in methodology provide the tools that allow the basic outlines of the building blocks of the human mind to come into focus. Early developing systems for representing objects, intentional agents, number, and causality are in place during the first months of life. The research exploits several types of behavior young infants are capable of, including the deployment of attention and manual search. The research yields evidence for rich representational capacities in infants. These measures also yield evidence that infant prelinguistic representations differ from linguistic representations and from later developing conceptual capacities in deep respects. One goal of the proposed studies is to characterize these differences. For example, although infants have several distinct systems of representation with numerical content, none has the representational power to express even the positive integers. A second goal of the proposed studies is to characterize the learning mechanisms through which human beings transcend their initial representational capacities. Training studies and cross-linguistic comparison studies will explore the role of language learning in this process. Unlike general retardation, many developmental disabilities (e.g., dyslexia, autism, dyscalculia, William Syndrome) implicate deficits in specific aspects of conceptual development. Successful treatments require an understanding of the cognitive mechanisms that underlie normal development. And any effort to relate brain development to behavioral development also requires a prior characterization of distinct systems of infant mental processing.